Congenital heart defects are the most common birth defects in the United States and cost the exchequer $1.4 billion each year for management. Understanding the signaling pathways that control normal cardiac development provides a molecular basis for the derangements that cause congenital heart disease. Comprehension of the specific genetic abnormalities that result in heart disease in individual patients will pave wa for personalized medicine. Furthermore, insights into normal cardiac development provide a framework to refine regenerative therapies aimed at repairing the injured heart. The outflow tract of the heart develops from complex interactions between cardiac progenitor cells that arise from the second heart field and neural crest cells. Whereas the role of the Notch receptor in appropriate outflow tract development is beginning to be recognized, the exact nature and source of the specific ligand is largely unknown. The central hypothesis of the attached research proposal is that Delta Like Ligand-4 (DLL-4) is expressed in the second heart field and mediates Notch-signaling during cardiac development. DLL-4 expression is critically required for appropriate incorporation of cardiac progenitor cells into the developing heart and its disruption results in abnormal outflow tract development and caudal pharyngeal arch artery defects. Preliminary data using mouse models show that in the absence of DLL-4 expression, there is reduced contribution of second heart field cells to the developing heart. The potential role of DLL-4 in the specification and integration of these progenitor cells to the heart will be evaluated in Aim 1 of the study. DLL-4 mutant embryos that survive to mid-gestation show mal-alignment of the outflow tract and double outlet right ventricle phenotype. The second Aim will characterize these phenotypic derangements and evaluate the potential molecular signals that are perturbed. In addition, mutant embryos also display a variety of aortic arch abnormalities. The third Aim will study this phenotype with particular focus on the interaction between second heart field DLL-4 and Notch receptors expressed by second heart field and neural crest cells. Successful completion of this research will fill a significant knowledge void in our comprehension of Notch-Delta signaling in cardiac outflow tract development. The candidate PI is an early career cardiac surgeon-scientist, who is dual trained as a clinical congenital heart surgeon and a basic science researcher with a PhD studying membrane protein signaling. His ultimate career goal is to synthesize the research and clinical training to provide a molecular basis to clinical congenital cardiac disease processes, thereby bridge the gap between bench and the bedside. This career development award will formalize focused mentored training in a new research direction being undertaken by this candidate. Towards this end, the proposal brings together a robust mentoring team that provides complimentary skills - cardiac developmental biology expertise, support for clinician-scientist, expertise in field of clinical congenital heart disease and established mentoring tools. The career development program integrates didactic education, research survival skills and support for academic advancement. Importantly, USC and the PI's department of Surgery present a solid culture for research excellence with qualified mentors, outstanding resources and a commitment to foster the growth of junior clinician-scientists.